The present invention relates to an apparatus for measuring the humidity of air and/or another gas, in particular, relates to a micro-wave hygrometer using micro-wave energy and providing an electrical output relating to the measured humidty.
It has been well known that the loss of the micro-wave energy due to the presence of water molecules (H.sub.2 O) in the air depends upon the air-pressure, the temperature of the air, and the partial pressure of the water vapour. Accordingly, the measurement of the micro-wave loss in the air, the air pressure, and the temperature of the air can provide the resultant partial pressure of the water vapour, that is to say, the humidity of the air, as proposed in the U.S. Pat. No. 2,792,548.
However, the U.S. Pat. No. 2,792,548 has the following disadvantages due to the use of a cavity resonator. The first disadvantage is that accurate measurement of the humidity is difficult because the pressure and/or the temperature might be different between in the cavity and in the air. The second disadvantage is that the measured humidity may have the secular variation since the metal wall of the cavity can be corroded by the moist air.
Another prior humidity meter is U.S. Pat. No. 3,946,308, which uses a dielectric resonator instead of the cavity resonator. However, it has the disadvantage that the measured humidity value drifts because of the presence of body which causes a loss in the micro-wave energy, located near the dielectric resonator. A body of an operator or another dielectric material might cause such drift.